Remove FF `jobs_api_keyset_pagination`"

What does this MR do and why?

Remove the feature flag jobs_api_keyset_pagination

Feature is 100% on all environments: https://gitlab.slack.com/archives/C101F3796/p1674166139817029

Screenshots or screen recordings

N/A

How to set up and validate locally

Calling jobs index endpoint using keyset pagination without any FF should still work and return a cursor

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #385940 (closed)